The present invention relates to a process for denitrating aqueous nitric acid and salt containing waste solutions.
In reprocessing irradiated and spent fuel and/or breeder elements there result aqueous, radioactive waste solutions which carry along a number of salts. Inter alia, these solutions contain actinide salts which must be removed from these solutions before further treatment of the waste solutions is effected, for example for the purpose of solidifying the radioactive fission nuclides. This removal of actinide salts could be done, for example, by precipitation with oxalic acid, if the solubility product of the actinide oxalates, e.g. plutonium oxalate, can be exceeded. However, this is possible only if the nitric acid, which is present in high concentration, is destroyed practically completely.
In the past, a series of denitration processes have been proposed to dispose of highly radioactive waste solutions. A few of these processes will be listed here:
R. C. Forsman and G. C. Oberg describe, in a US-AEC report from the Hanford Works, HW-79622, October 1963, entitled "Formaldehyde Treatment of Purex Radioactive Waste," a denitration with formaldehyde.
S. Drobnik destroys nitric acid with formic acid, as described in German Pat. No. 1,935,273 and corresponding U.S. Pat. No. 3,673,086.
L. A. Bray and E. C. Martin disclose denitration with sugar in U.S. Pat. No. 3,158,577.
W. Boccola and A. Donato denitrate with phosphorus, as disclosed in German Pat. No. 2,125,915.
H. Richter and H. Sorantin describe, in a report of the Austrian Studiengesellschaft fur Atomenergie GmbH (in translation, Study Group for Atomic Energy) (Seibersdorf) SGAE Report No. 2252 ST 23/74, March 1974, the destruction of excess nitric acid in radioactive waste solutions with the aid of glycerin and the subsequent solidification of the residue as alkyd resin.
The above-listed processes have the following drawbacks:
In denitration with formaldehyde or formic acid, the nitric acid solution and denitration reagent cannot be mixed before the reaction since otherwise the denitration reaction would be too violent during heating. The denitration reagent must be added in measured quantities during the reaction. Separation of excess reagent is fraught with problems.
In denitration with sugar, the sugar is added as an aqueous solution, and this adds to the volume of the waste solution.
In denitration with phosphorus, the nonvolatile phosphoric acids are formed, inter alia, which remain in the denitrated solution.
In denitration with glycerin, the reaction exhibits an induction period as a function of temperature. Here again, the denitration reagent is added in measured quantities during the reaction.